The assholes at Lumileds are too good to give out actual dimensions of the star PCB on their datasheets. Looking at some Cree stars I have from some other unknown Pacific Rim manufacturer, they have roughly the same footprint as some old Luxeon III stars I have.

I'd be careful about running those things at more than ~350mA - Luxeon stars are well-engineered for heat transmission while almost all of the Cree stars aren't.